FIG. 5 is a perspective view illustrating the construction of a prior art SAW convolver, in which reference numeral 1 is a piezoelectric layer; 2 is an insulating layer; 3 is a semiconductor layer; 4 is a gate electrode; 5 are input electrodes, e.g. interdigital electrodes; 6 is a rear surface electrode; 7 are input terminals and 8 is an output terminal. A convolution signal of input signals applied to the two interdigital electrodes 5 is taken out through the gate electrode 4.
The construction indicated in FIG. 5 has an advantage that the convolution efficiency of the convolver is very high.
However, in the construction indicated in FIG. 5, in addition to the convolution signal between the signals inputted in the two interdigital electrodes (hereinbelow abbreviated to I.D.T) 5, unnecessary signals called self convolution signals are produced. The self convolution signal is a signal produced by the fact that a surface wave produced by an I.D.T is reflected by the other I.D.T opposite thereto, and corresponds to a convolution signal of itself. FIG. 6 shows this aspect. It will be clearly understood from FIG. 6 that in addition to an object convolution signal P.sub.out between surface acoustic waves S.sub.1 and S.sub.2 due to input signals P.sub.1 and P.sub.2, other convolution signals are produced also between S.sub.1 and its reflected wave S.sub.1r as well as S.sub.2 and its reflected wave. The later two signals are self convolution signals.
Now such self convolution signals become spurious noise against the output of the convolver and have unfavorable influences thereon in that they reduce the dynamic range of the convolver. Particularly, in the conventional construction as indicated in FIG. 5, there is a drawback that these influences become remarkable, when the gate electrode 4 is short. In this regard, more in detail, refer to the following literature;
Literature [1] S. Minagawa, et al., "Efficient ZnO-SiO.sub.2 -Si Sezawa Wave Convolver", IEEE Trans Sonics Ultrason, vol.SU-32, No. 5, September 1985, pp670-674. PA1 (Literature [3]), etc. PA1 Literature [2]) I. Yao, "High-performance Elastic Convolver with Parabolic Horns", Proc. 1980 IEEE Ultrason Symp., 1980 pp37-42 PA1 Literature [3] C. L. West, "SAW convolver employing unidirectional transducers for improved efficiency":, Proc. 1982, IEEE Ultrason Symp., 1982, pp119-123.
As the general method for reducing such influences of the self convolution signal, there are known e.g. a method for using a convolver having a dual gate (Literature [2]), a method, for using a unidirectional transducer
However, the former method has as a disadvantage that the area of the SAW element is large and that the external circuit connected thereto is complicated, while the latter method has as a disadvantage that the area of the SAW element is likewise large and that it is difficult to enlarge the frequency band width thereof.